When brush or spray applie this high buil . Primer Surfacer is ideal for improving adhesion of the final topcoat and enhancing paint’s color when brushed or sprayed to metal or fiberglass surfaces. Buy SEM 391Flexible Primer Surfacer – oz. FREE DELIVERY possible on eligible purchases. This is an area where many make the mistake of applying a primer surfacer (high buil sandable primer) directly to bare metal.